1. Field of the Invention
The present invention relates to an antistatic coat suitable for coating a surface, in particular, a friction surface of a base body. In addition, the present invention relates to a thermal transfer sheet having an excellent antistatic property on the back surface side thereof, on which a friction is inflicted by a heating member such as a thermal head or the like. Furthermore, the present invention relates to an antistatic agent effectively used for manufacturing the above antistatic coat and the thermal transfer sheet.
2. Description of the Related Art
There have been various problems caused by the accumulation of static electric charges in such various film shaped products and resinous articles as a packaging material for electronic parts, an OHP film, a thermal transfer sheet or the like. There are proposed various methods for preventing the accumulation of static electric charges in the film shaped products. A cationic surface active agent is widely used as an antistatic agent. Although the above method using the cationic surface active agent is convenient and easily applicable method, the durability of the antistatic effect is not sufficient.
A thermal transfer sheet is explained as the principal example of the film shaped products, hereinbelow.
As a thermal transfer sheet, there are known a sublimation type thermal transfer sheet in which a dye layer comprising a sublimation dye and a binder is disposed on one surface of a substrate film such as a polyester film or the like, and a heat fusion type thermal transfer sheet in which an ink layer comprising a wax and a pigment, in place of the above dye layer, is disposed on the substrate film. Such a thermal transfer sheet is image-wise heated from its back surface side by the thermal head to transfer the dye of the dye layer or the ink of the ink layer to the image-receiving material, thus forming an image.
In case that an image is formed by the thermal head with the use of the above thermal transfer sheet, and when the substrate film is made of a thermoplastic resin like a polyester film or the like, a surface electrical resistance of the surface contacted by the thermal head is as high as at least 10.sup.14 (.OMEGA.), thus inducing static electricity in the process of forming the image to accumulate electric charges thereon.
When the electric charges are accumulated on the thermal transfer sheet, dusts or the like are attracted to the surface thereof, as a result, the dusts or the like are attached onto the thermal head, thus lowering the resolution of the formed image. In addition, similarly, the electric charges are accumulated on the image-receiving material such as papers, lowering a conveying ability of the image-receiving material. Furthermore, in worse case, there may occur sparks when the thermal transfer roll or the image-receiving material is replaced or inserted. In addition, there may be cases that human bodies are shocked.
As a method to overcome the above problems, there is known a method that an antistatic layer is formed on the back surface of the thermal transfer sheet. However, when the antistatic layer is formed with the use of a surface active agent or the like, there is caused a sticking on the thermal transfer sheet. In addition, there is the following problem when the thermal transfer sheet is rolled, the antistatic agent is transferred to the coloring material layer, or in reverse, the coloring material is transferred to the back surface of the thermal transfer sheet. Furthermore, together with the above problems, there is a further problem that the antistatic effect of the antistatic layer lowers as the time passes.
As another method, there is a method in which a conductive layer is formed with the combined use of a binder and a conductive agent like a conductive carbon black. However, according to the above method, the thermal head severely wears, in addition, when the thermal transfer sheet except a black color one is used, the image-receiving material becomes black as a whole, thus deteriorating appearance thereof, and furthermore, it becomes difficult to detect the detecting mark formed on the thermal transfer sheet.
As further another method, there is proposed a method in which an antistatic layer is formed with the use of an acrylic resin containing a quaternary ammonium salt (refer to Japanese Patent Provisional Publication No. 2-182,491). However, according to the above method, since the quaternary ammonium salt comprises chlorine ion or bromine ion in general, the thermal head is remarkably corroded by such a halogen ion, thus causing another problem.
Furthermore, since the thermal head is heated at a high temperature, the thermal head is stuck to the substrate film, thus deteriorating an excellent mobility of the thermal head to cause breakage and wrinkles in the thermal transfer sheet.
As a method to solve the above problems, there is known a method in which a back surface layer comprising a heat resistance resin is formed on the contact surface to the thermal head. However, it is difficult in the above method to form such a thin film with the use of the heat resistance resin as not to lower a sensitivity thereof. Furthermore, even if such a thin film can be formed, it cannot afford a sufficient slipping ability, and resulting in an insufficient mobility of the thermal head.
On the other hand, there is known a method in which a slip layer is formed on the outermost surface of the back surface with the use of a silicone oil or silicone wax. However, since the silicone oil or silicone wax has a lower film strength, the slip layer thus formed is scraped off by the thermal head, and thus scraped slip layer deposits on the surface of the thermal head as the head grime, resulting in a poor printing.
In addition, it is impossible to form both of the antistatic layer and slip layer at the outermost position of the back surface.
More specifically, when the antistatic layer is formed on the outermost surface of the friction surface such as the contact surface with the thermal head, the remarkably excellent antistatic effect can be obtained. However, on the other hand, the durability of the antistatic effect is insufficient, and, the sticking on the surface of the thermal head and the contamination of the antistatic layer are caused. In addition, the bad influence to the other articles such as the image receiving sheet, the thermal head or the like was inflicted by the antistatic layer, for example, the contamination of the coloring material layer, the wear or corrosion of the thermal head. Furthermore, it is necessary to provide the friction surface of the products with not only antistatic property, but also other functions such as heat resistance and slipping ability. However, when the antistatic layer is formed on the outermost surface of the friction surface, the other functions which is necessary for the friction surface are sacrificed.